This invention relates generally to methods and products for reducing print image defects in scavengeless development systems. More particularly, the invention relates generally to methods and products for reducing visible image defects resulting from powder accumulation on electrode wires in such scavengeless development systems. The invention utilizes toner and developer compositions having increased toner fines, i.e., increased amounts and proportions of fine toner particles, to overcome this image defect.
In electrophotography, a photoreceptor containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The photoreceptor is then exposed to a pattern of activating electromagnetic radiation, such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided toner particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the photoconductor to a support, such as transparency or paper. This imaging process may be repeated many times.
Various toner compositions for such a printing system are well known in the art, and have been produced having a wide range of additives and constituent materials. Generally, however, the toner particles include a binding material such as a resin, a colorant such as a dye and/or a pigment, and any of various additives to provide particular properties to the toner particles.
Development may be interactive or non-interactive depending on whether toner already on the image may or may not be disturbed or removed by subsequent development procedures. Sometimes the terms scavenging and non-scavenging or scavengeless are used interchangeably with the terms interactive and non-interactive. Non-interactive or scavengeless development is most useful in color systems when a given color toner must be deposited on an electrostatic image without disturbing previously applied toner deposits of a different color, or cross-contaminating the color toner supplies.
Apparently useful non-interactive development methods known in the art work by generating a powder cloud in the gap between the photoreceptor and another member that serves as a development electrode. It is generally observed that this gap should be as small as possible, as small as 0.010 inches or smaller. Generally, the larger the gap, the larger become certain image defects in the development of fine lines and edges. The lines do not develop to the correct width, lines near solid areas are distorted, and the edges of solids are softened, especially at corners. It is believed that these defects are due to arches in the image electric fields over lines and at the edges of solid areas. In these arches electric field lines from image charges loop up and return to the photoreceptor ground plane instead of reaching across through the cloud to the development electrode. Defects result because toner in the cloud moves generally along field lines and cannot cross them into the arches, with the result that the deposited toner distribution does not correspond to image charge distribution. Defects due to field arches are less serious in interactive two component development because toner is carried into the arches by carrier particles. Nor are they very serious in interactive single component development exemplified by U.S. Pat. No. 4,292,387 to Kanbe et al. because a strong, cross-gap AC field is superposed which overcomes the aforementioned field arch patterns.
In non-scavenging systems cross gap AC fields are also applied. However, it is important to realize that if such fields are made too strong, the system will become interactive due to toner impact on already developed images. Thus a system may image well at strong fields and develop non interactively at weak fields, but not do both simultaneously. The development electrode and its role in determining electric field structure is described, for example by H. E. J. Neugebauer in Xerography and Related Processes, Dessauer and Clark, Focal Press, 1965. Powder cloud development is described, for example, in the paper “High Sensitivity Electrophotographic Development” by R. B. Lewis and H. M. Stark in Current Problems in Electrophotography, Berg and Hauffe, Walter de Gruyter, Berlin, 1972.
U.S. Pat. No. 4,868,600 to Hays et al discloses a non-interactive development system wherein toner is first developed from a two-component developer onto a metal-cored donor roll and thereafter disturbed into a powder cloud in the narrow gap between the donor roll and an electrostatic image. Development fields created between the donor roll core and the electrostatic image harvest some of the toner from the cloud onto the electrostatic image, thus developing it without physically disturbing it. In this method the powder cloud generation is accomplished by thin, AC biased wires strung across the process direction and within the development gap. The wires ride on the toner layer and are biased relative to the donor roll core. The method is subject to wire breakage and to the creation of image defects due to wire motion, and these problems increase as the process width is increased. In this system it has been found important for image defect reduction to minimize the gap between the donor and the surface of the electrostatic image in order to create a close development electrode. Gap spacings of about 0.010 inches are characteristic. They would be smaller were it practical to maintain the necessary tolerances.
U.S. Pat. No. 4,557,992 to Haneda et al. describes a non-interactive magnetic brush development method wherein a two component developer employing magnetically soft carrier materials is carried into close proximity to an electrostatic image and caused to generate a powder cloud by the developer motion, sometimes aided by an AC voltage applied across the gap between the brush and the ground plane of the electrostatic image. Cloud generation directly from the surfaces of a two component developer avoids the problems created by wires. However, in practice such methods have been speed limited by their low toner cloud generation rate.
U.S. Pat. No. 5,409,791 to Kaukeinen et al. describes a non-interactive magnetic brush development method employing permanently magnetized carrier beads operating with a rotating multipole magnet within a conductive and nonmagnetic sleeve. Magnetic field lines form arches in the space above the sleeve surface and form chains of carrier beads. The developer chains are held in contact with the sleeve and out of direct contact with the photoreceptor by gradients provided by the multipole magnet. As the core rotates in one direction relative to the sleeve, the magnetic field lines beyond the sleeve surface rotate in the opposite sense, moving chains in a tumbling action which transports developer material along the sleeve surface. The strong mechanical agitation very effectively dislodges toner particles generating a rich powder cloud which can be developed to the adjacent photoreceptor surface under the influence of development fields between the sleeve and the electrostatic image.
Nevertheless, there continues to be a need for improved development processes. In particular, there continues to be a need for improved development processes such that print image defects can be reduced or eliminated, thereby to provide more desirable printed images.